Is there an equivalent to Polygon Colliders 2D but for 3D?

Is it possible to take a Box Collider 3D and to change the position of its vertices?

I would like to make a simple shape that resembles the leg of my character, but without using a mesh collider for performance reasons.

For 3D physics, there are 5 options for colliders: Sphere, Box, Capsule, Compound, and Mesh. Compound is just multiple basic colliders on one object. Unity has a basic tutorial about colliders here:

http://unity3d.com/learn/tutorials/modules/beginner/physics/colliders

Without knowing the exact situation and model, my suggestion is to use a Compound setup with multiple Box/Capsule Colliders to simulate the leg shape. For instance have one covering the upper leg and another for the lower leg. This would allow the model to bend the legs and still be relatively accurate.

If you really need more accuracy, then a Mesh Collider is probably your only choice. Usually when a Mesh Collider is needed, it’s a simplified version of the actual model so it has a lot few vertices than the visible model. You can still have multiple Mesh Colliders, one for upper leg and one for lower.